I. Field of the Invention
The present invention relates to wireless communication and radar systems. More specifically, the present invention relates to a novel and improved antenna array processor that controls beam-forming and scanning operations and that also introduces a new spread spectrum technique.
II. Description of the Related Art
Multiple access communication techniques include time-division multiple access (TDMA), frequency division multiple access (FDMA), amplitude modulation, and spread spectrum. Spread spectrum techniques provide some improvements over the other multiple access techniques depending on the type of spread spectrum used. Spread spectrum techniques are based on the principle of expanding a transmitted baseband signal in frequency. This achieves superior interference-rejection by utilizing high process gain to reduce noise and interference in the received signal.
There are four basic types of spread spectrum. Frequency-hopping spread spectrum (FHSS) is a well-known technique that provides effective rejection of narrow-band jamming interference and mitigates near-far interference. Chirped FM spread spectrum is a technique used primarily in radar systems. Orthogonal frequency division multiplexing (OFDM) is used to spread high data-rate information streams into multiple low data-rate streams carried on separate carrier frequencies. Direct sequence CDMA (DS-CDMA) is particularly useful in multiple access communication systems because it allows for very efficient use of the frequency spectrum and provides for improved frequency reuse. There are also hybrid techniques that combine various aspects of the four basic spread spectrum types. Most notable are frequency-hopped direct sequence, time-division direct sequence, and orthogonal frequency CDMA (also known as multi-frequency CDMA or MF-CDMA).
Frequency reuse is the process of using the same frequency in two separate geographic regions for two distinct communication links. Frequencies can be reused provided that the two regions are attenuated or isolated from each other by a minimum value for signal rejection by user receivers in each region. U.S. Pat. No. 4,901,307 describes the process of creating marginal isolation, which provides an increase in frequency reuse in DS-CDMA systems. In DS-CDMA, even small reductions in the overall power level of the system allow for increased system capacity. One particularly effective method for creating isolation and improving frequency reuse is spatial division multiple access (SDMA). SDMA applications to multiple access communication systems including adaptive array processing are discussed in U.S. Pat. No. 5,642,353, U.S. Pat. No. 5,592,490, U.S. Pat. No. 5,515,378, and U.S. Pat. No. 5,471,647. In addition to frequency reuse, antenna arrays also provide increased processing gain and improved interference rejection.
The advantage to using adaptive antenna arrays for DS-CDMA communications is that adaptive antenna arrays could provide significant improvements in range extension, interference reduction, and capacity increase. To identify a particular user, a DS-CDMA system demodulates Walsh codes after converting the received signal from RF to digital. Therefore, an adaptive antenna array requires information about the user codes from CDMA radio, or it needs to demodulate many different incoming RF signals to track mobile users. These methods are complex processes and are more difficult to implement than the tracking of users in non-CDMA systems. Major changes in CDMA radio architecture are required to implement adaptive array processing. These changes may be the major obstacle for adaptive array deployment in the near future.
Phased array antenna systems employ a plurality of individual antennas or subarrays of antennas that are separately excited to cumulatively produce an electromagnetic wave that is highly directional. The radiated energy from each of the individual antenna elements or subarrays is of a different phase so that an equiphase beam front, or the cumulative wave front of electromagnetic energy radiated from all of the antenna elements in the array, travels in a selected direction. The difference in phase or timing between the antenna's activating signals determines the direction in which the cumulative wave front from all of the individual antenna elements is transmitted. Analysis of the phases of return beams of electromagnetic energy detected by the individual antennas in the array similarly allows determination of the direction from which a return beam arrives.
Beamforming, which is the adjustment of the relative phase of the actuating signals for the individual antennas, can be accomplished by electronically shifting the phases of the actuating signals. Beamforming can also be performed by introducing a time delay in the different actuating signals to sequentially excite the antenna elements which generate the desired direction of beam transmission from the antenna. However, phase-based electronically controlled phased array systems are relatively large, heavy, complex, and expensive. These electronic systems require a large number of microwave components (such as phase shifters, power splitters, and waveguides) to form the antenna control system. This arrangement results in a system that is relatively lossy, electromagnetically sensitive, hardware-intensive, and has a narrow tunable bandwidth.
Optical control systems can be advantageously used to create selected time delays in actuating signals for phased array systems. Such optically generated time delays are not frequency dependent and thus can be readily applied to broadband phased array antenna systems. For example, optical signals can be processed to establish the selected time delays between individual signals, thus causing the desired sequential actuation of the transmitting antenna elements. The optical signals can then be converted to electrical signals, such as by a photodiode array. Different types of optical architectures have been proposed to process optical signals that generate selected delays. Examples of these architectures are fiber optic segments of different lengths for routing the optical signals; deformable mirrors for physically changing the distance light travels along a reflected path before being converted to an electrical signal; and free space propagation based delay lines, which typically incorporates polarizing beam splitters and prisms.
U.S. Pat. No. 5,117,239 and U.S. Pat. No. 5,187,487 describe a system that creates a cluster of optical beams coupled into individually-controlled pixels of a spatial light modulator (SLM). The SLM provides selectable phase shifts to each of the beams. Some optical delay devices, such as U.S. Pat. No. 5,461,687, utilize the refractive properties of different wavelengths of light to provide individually controlled phase shifting of wavelength-multiplexed light. Although optical processing offers great improvements over radio frequency (RF) and digital array processing, current optical processing approaches merely replace microwave components with optical components without reducing the complexity of the system. For example, an optical system having a number N of array elements requires N phase-shifters and N associated phase-shifter control systems. Some devices, such as Rotman lenses, are designed to reduce or eliminate the need for adjustable phase shifters. However they increase system complexity and size by introducing complex elements and systems as well as by introducing additional detectors.
Several optical systems that exhibit unusual properties have been built, but their application to phased array signal processing had been overlooked. In the Optics Letters article “Broadband Continuous Wave Laser,” applicant described a laser design that utilizes a traveling-wave frequency-shifted feedback cavity (FSFC) to circulate light through a gain medium. Light circulating through the FSFC is frequency shifted by an acousto-optic modulator (AOM) upon each pass through the cavity. A unique characteristic of this cavity is that, unlike a Fabry-Perot cavity, it does not selectively attenuate signal frequencies. In the thesis “A New Method for Generating Short Optical Pulses,” applicant describes how an optical signal propagating through a FSFC is spread in frequency to generate broadband lasing, where the amount of frequency spreading is proportional to the number of times that light circulates through the cavity. In the Applied Physics Letters article “Optical Pulse Generation with a Frequency Shifted Feedback Laser,” applicant describes an interference condition in which the broadband output of the laser produces short optical pulses, which have a frequency that is related to the RF shift frequency of the AOM. The time-domain characteristics of these optical pulses are similar to RF pulse-radio emissions.
Although pulse-radio systems are well known in the art, they are not well suited for commercial applications. Pulse-radio is a time-domain system that produces broadband radiation as a natural artifact resulting from the generation of short-duration pulses. Broad bandwidth, hence large effective processing gain, makes pulse radio ideal for covert communications. However, its broad bandwidth, particularly the portion occupying the low-frequency ranges of the RF spectrum, makes proposed commercial pulse-radio systems unlikely candidates for FCC approval. The short pulse width of pulse-radio signals makes Rake reception very difficult. A Rake receiver used in a pulse-radio system would require an extraordinary number of taps, on the order of the pulse repetition rate divided by the pulse width.